1. Field of the Invention
This invention relates generally to high-frequency alternators used to generate electrical power, and more specifically to power-frequency generators suitable for use with variable shaft speed, and, more particularly, to three-phase power-frequency generators suitable for use with variable shaft speed.
2. Description of the Prior Art
Power-frequency electrical generation provides alternating current and voltage at power-frequencies common to utility “grid” networks, (generally 60 Hz alternating current (AC) in the United States, or 50 Hz AC in many other areas). Devices for this purpose can be single phase, or, more commonly in larger grid connected networks, three-phase systems where three individual single phases operate in concert but each phase is displaced by one third of a power-frequency electrical cycle from the other two phases.
Power-frequency electrical generation generally relies upon either synchronous devices, where the shaft speed is directly tied to the required output electrical frequency, or relies on “asynchronous” induction devices where the shaft speed “slips” within a very small range relative to the synchronous speed. Neither of these approaches allows for true variable speed operation, which would be convenient for highly variable power sources such as wind or hydro-power. Furthermore, these approaches do not allow for variable speed operation to maximize the fuel efficiency of power sources such as internal combustion engines operating at partial load.
Another variable speed approach, the doubly-fed or wound rotor induction generator (WRIG) can be economically employed at the multi-megawatt power level. This approach requires a large portion of the power (approximately 15-25%) to be carried to three-phase AC windings on the rotor for excitation purposes. The power connection to the rotor is usually accomplished by slip rings and the rotor power is a variable frequency three-phase excitation that must be carefully controlled to compensate for the variable speed of the rotor shaft. This rotor power can be provided by a high power variable-frequency converter. Both the inductive generators and WRIG systems rely on a grid interconnection to provide external excitation currents of substantial magnitude.
Another approach to power-frequency electrical generation is to use high-frequency alternators to generate high-frequency alternating current that is rectified into a direct current (DC) voltage supply that is then re-formed by switching power electronics (generally inverters and power-frequency converters) into power-frequency AC electrical power. This approach allows power-frequency generation using alternators with variable shaft speed. Disadvantages of this approach include the cost, efficiency penalties, heat loss and complexities of the rectification and high-frequency switching processes of the power electronics.
A distinct “field-excitation” approach, is detailed by Hilgendorf in U.S. Pat. No. 3,916,284 and Tupper in U.S. Pat. No. 6,051,959. In this approach the field of a poly-phase high-frequency alternator is modulated by a sinusoidal excitation current, at the desired power-frequency, and shapes the rectified output into a rectified power-frequency output waveform. The rectification can be done by simple diodes or thyristors sometimes referred to as silicon-controlled rectifiers (SCRs) using natural commutation, in contrast to the so called “hard switching” of inverters, thereby minimizing switching losses and stresses on the power electronics. Minimal additional electronics are needed for commutation to unfold the rectified sinusoidal voltage into the desired bipolar power-frequency sinusoidal (AC) waveform and this commutation can be done as the output currents approach zero, resulting in minimal losses. Using the resonant excitation techniques of U.S. Pat. No. 6,051,959, the field excitation approach can be configured to consume minimal power for excitation. Commercial systems of this type are available in which excitation power requires about 2% of the output power. The low excitation power requirement enhances the efficiency of the system, and, importantly, reduces the waste heat and the attendant dissipation issues. Since a grid connection is not required for excitation, these systems can be used for stand-alone operation. These field excitation approaches avoid many of the disadvantages of the inverter based systems while maintaining the variable speed advantages of the high-frequency alternator approach.
To date, these field excitation approaches have focused on single-phase power-frequency generation devices making this technique mostly suitable for moderate power levels for stand-alone and mobile applications not generally intended to be connected to the utility grid.
In U.S. Pat. No. 6,133,669, Tupper shows the importance of low loss magnetic cores to accomplish AC excitation of the field in high-frequency alternators. Further, in U.S. Pat. No. 6,177,746, Tupper and Wood show how a brushless high-frequency alternator can be arranged with axial air gaps and essentially axial magnetic flux flow to provide the low loss magnetic core and the low output inductance required for successful power-frequency modulation of the field for a single phase device. However, the axial magnetic flux flow of the latter device imposes the need for an axial return path for the magnetic flux, increasing the size and complexity of the alternator device.